hot rolled thin cast strip product and method for making the same

ABSTRACT

A hot rolled steel strip made by the steps including assembling a twin roll caster, forming a casting pool of molten steel of such composition that the cast strip produced comprises by weight, greater than 0.25% and up to 1.1% carbon, between 0.40 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, counter rotating the casting rolls to solidify metal shells and forming a steel strip, hot rolling the steel strip such that mechanical properties at 10% and 35% reduction are within 10% for yield strength, tensile strength and total elongation, and coiling the hot rolled steel strip at a temperature between 550 and 750° C. to provide a majority of the microstructure comprising pearlite, along with bainite and acicular ferrite. The steel may have a free oxygen content between 5 and 50 ppm or between 25 and 45 ppm.

This application claims priority to and the benefit of U.S. PatentApplication 61/154,248, filed on Feb. 20, 2009, which is incorporatedherein by reference.

BACKGROUND AND SUMMARY

In a twin roll caster, molten metal is introduced between a pair ofcounter-rotated, internally cooled casting rolls so that metal shellssolidify on the moving roll surfaces, and are brought together at thenip between them to produce a solidified strip product, delivereddownwardly from the nip between the casting rolls. The term “nip” isused herein to refer to the general region at which the casting rollsare closest together. The molten metal is poured from a ladle through ametal delivery system comprising a tundish and a core nozzle locatedabove the nip to form a casting pool of molten metal, supported on thecasting surfaces of the rolls above the nip and extending along thelength of the nip. This casting pool is usually confined betweenrefractory side plates or dams held in sliding engagement with the endsurfaces of the rolls so as to dam the two ends of the casting poolagainst outflow. The cast strip is typically directed to a hot rollingmill where the strip is hot reduced by 10% or more.

In certain steel applications, medium and high carbon pearlitic steelshave been desired, such as for banding or strapping, and certainapplications such as wall-ties for construction needing high strengthand controlled toughness. Pearlite typically offers increased strengthwith reduced impact toughness. In the past, pearlitic microstructureswere obtained by hot rolling and slow cooling medium and high carbonsteel strip having thicknesses greater than about 3.0 millimeter.Obtaining thinner strip such as 1.0 to 1.5 millimeter required multiplerounds of cold rolling and annealing that coarsened if not eliminatedthe pearlite, reducing the strength of the steel.

A hot rolled steel strip is disclosed made by the steps comprising:

-   -   assembling an internally cooled roll caster having laterally        positioned casting rolls forming a nip between them, and forming        a casting pool of molten steel supported on the casting rolls        above the nip and confined adjacent the ends of the casting        rolls by side dams, the molten steel of such composition that        hot rolled thin cast strip produced has a composition comprising        by weight, greater than 0.25% and up to 1.1% carbon, between 0.4        and 2.0% manganese, between 0.05 and 0.50% silicon, less than        0.01% aluminum,    -   counter rotating the casting rolls to solidify metal shells on        the casting rolls as the casting rolls move through the casting        pool, and    -   forming from the metal shells downwardly through the nip between        the casting rolls a steel strip,    -   hot rolling the steel strip such that mechanical properties at        10% and 35% reduction are within 10% for yield strength, tensile        strength and total elongation; and    -   coiling the hot rolled steel strip at a temperature between 550        and 750° C. to provide a majority of the microstructure        comprising pearlite, along with bainite and acicular ferrite in        the microstructure.

Alternatively, the step of hot rolling may be such that mechanicalproperties at 15% and 35% reduction are within 10% for yield strength,tensile strength and total elongation. In another alternative, themechanical properties are within 10% throughout the range from 15% to35% reduction for yield strength, tensile strength and total elongation.Alternatively, mechanical properties may be within 10% throughout therange from 10% to 35% reduction for yield strength, tensile strength andtotal elongation.

The molten steel cast may have a free oxygen content between 5 and 50ppm or content between 25 and 45 ppm. The total oxygen content may bemore than 20 and typically less than 100 ppm.

The molten steel may have a composition such that the manganese contentof the composition of the hot rolled steel strip is between 0.9 and 1.3%by weight. Alternatively or in addition, the molten steel may have acomposition such that the niobium content of the composition of the hotrolled steel strip is between about 0.01% and 0.2%. Alternatively or inaddition, he composition of molten steel may be such that thecomposition of the hot rolled steel strip may have a compositioncomprising at least one element selected from the group consisting ofmolybdenum between about 0.05% and about 0.50%, vanadium between about0.01% and about 0.20%, and a mixture thereof. The hot rolled steel stripmay have a tensile strength of at least 800 MPa after hot rollingreductions of 15% and 35%.

The steps of making the hot rolled steel strip may include hot dipcoating the hot rolled steel strip to provide a coating of zinc or azinc alloy.

Alternatively, a hot rolled steel strip may be made by the stepscomprising:

-   -   assembling an internally cooled roll caster having laterally        positioned casting rolls forming a nip between them, and forming        a casting pool of molten steel supported on the casting rolls        above the nip and confined adjacent the ends of the casting        rolls by side dams, the molten steel of such composition that        hot rolled thin cast strip produced has a composition comprising        by weight, greater than 0.25% and up to 1.1% carbon, between 0.5        and 2.0% manganese, between 0.05 and 0.50% silicon, less than        0.01% aluminum,    -   counter rotating the casting rolls to solidify metal shells on        the casting rolls as the casting rolls move through the casting        pool, and    -   forming from the metal shells downwardly through the nip between        the casting rolls a steel strip,    -   hot rolling the steel strip such that mechanical properties at        10% and 35% reduction are within 10% for yield strength, tensile        strength and total elongation; and    -   coiling the hot rolled steel strip at a temperature between 200        and 550° C. to provide a majority of the microstructure        comprising bainite, martensite, and acicular ferrite, and having        less than 5% polygonal ferrite in the microstructure.

Alternatively, the step of hot rolling may be such that mechanicalproperties at 15% and 35% reduction are within 10% for yield strength,tensile strength and total elongation. In yet another alternative, themechanical properties are within 10% throughout the range from 15% to35% reduction for yield strength, tensile strength and total elongation.Alternatively, mechanical properties may be within 10% throughout therange from 10% to 35% reduction for yield strength, tensile strength andtotal elongation.

The molten steel cast may have a free oxygen content between 5 and 50ppm or content between 25 and 45 ppm. The total oxygen content may bemore than 20 and typically less than 100 ppm.

The molten steel may have a composition such that the manganese contentof the composition of the hot rolled steel strip is between 0.9 and 1.3%by weight. Alternatively or in addition, the molten steel may have acomposition such that the niobium content of the composition of the hotrolled steel strip is between about 0.01% and 0.2%. Alternatively or inaddition, he composition of molten steel may be such that thecomposition of the hot rolled steel strip may have a compositioncomprising at least one element selected from the group consisting ofmolybdenum between about 0.05% and about 0.50%, vanadium between about0.01% and about 0.20%, and a mixture thereof. The hot rolled steel stripmay have a tensile strength of at least 1100 MPa after hot rollingreductions of 15% and 35%. Alternatively, the hot rolled steel strip mayhave a tensile strength between about 1100 and 1700 MPa after hotrolling reductions of 15% and 35%.

The steps of making the hot rolled steel strip may include hot dipcoating the hot rolled steel strip to provide a coating of zinc or azinc alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a strip casting installation incorporating an in-linehot rolling mill and coiler;

FIG. 2 illustrates details of the twin roll strip caster;

FIG. 3A is an optical micrograph of a comparative 0.19% carbon steelcoiled at 600° C. and hot rolling reduction of 18%;

FIG. 3B is an optical micrograph of a comparative 0.19% carbon steelcoiled at 600° C. and hot rolling reduction of 38%;

FIG. 4A is an optical micrograph of a 0.46% carbon steel showing amicrostructure including grain boundary ferrite and fine lamellarpearlite;

FIG. 4B is an optical micrograph of a 0.46% carbon steel showingintragranular acicular ferrite;

FIG. 5 is an electron microscope thin foil image of 0.46% carbon steelshowing fine lamellar structure;

FIG. 6 is a graph showing the effect of amount of carbon on the tensilestrength, yield strength, and elongation for test samples between 0.3and 0.4% carbon;

FIG. 7 is a graph showing the effect of hot rolling reduction on thetensile strength, yield strength, and elongation over reduction betweenabout 15% and 23%, and

FIG. 8 is a graph showing the effect of coiling temperature on thetensile strength, yield strength, and elongation for coilingtemperatures between about 1180 and 1300° F. (about 640 and 700° C.).

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates successive parts of strip caster for continuouslycasting steel strip. FIGS. 1 and 2 illustrate a twin roll caster 11 thatcontinuously produces a cast steel strip 12, which passes in a transitpath 10 across a guide table 13 to a pinch roll stand 14 having pinchrolls 14A. Immediately after exiting the pinch roll stand 14, the strippasses into a hot rolling mill 16 having a pair of reduction rolls 16Aand backing rolls 16B where the cast strip is hot rolled to reduce adesired thickness. The hot rolled strip passes onto a run-out table 17where the strip may be cooled by convection and contact with watersupplied via water jets 18 (or other suitable means) and by radiation.The rolled and cooled strip is then passes through a pinch roll stand 20comprising a pair of pinch rolls 20A and then to a coiler 19. Finalcooling of the cast strip takes place after coiling.

As shown in FIG. 2, twin roll caster 11 comprises a main machine frame21, which supports a pair of laterally positioned casting rolls 22having casting surfaces 22A. Molten metal is supplied during a castingoperation from a ladle (not shown) to a tundish 23, through a refractoryshroud 24 to a distributor or moveable tundish 25, and then from thedistributor 25 through a metal delivery nozzle 26 between the castingrolls 22 above the nip 27. The molten metal delivered between thecasting rolls 22 forms a casting pool 30 above the nip. The casting pool30 is restrained at the ends of the casting rolls by a pair of sideclosure dams or plates 28, which are pushed against the ends of thecasting rolls by a pair of thrusters (not shown) including hydrauliccylinder units (not shown) connected to the side plate holders. Theupper surface of casting pool 30 (generally referred to as the“meniscus” level) usually rises above the lower end of the deliverynozzle so that the lower end of the delivery nozzle is immersed withinthe casting pool 30. Casting rolls 22 are internally water cooled sothat shells solidify on the moving roller surfaces as they pass throughthe casting pool, and are brought together at the nip 27 between them toproduce the cast strip 12, which is delivered downwardly from the nipbetween the casting rolls.

The twin roll caster may be of the kind that is illustrated anddescribed in some detail in U.S. Pat. Nos. 5,184,668 and 5,277,243 orU.S. Pat. No. 5,488,988, or U.S. patent application Ser. No. 12/050,987.Reference may be made to those patents for appropriate constructiondetails of a twin roll caster appropriate for use in an embodiment ofthe present invention.

The composition of the hot rolled steel strip has greater than 0.25%carbon and up to about 1.1% carbon to provide desired strength andmicrostructure having a thickness less than 3.0 mm, and may be less than2.5 mm. Alternatively, the steel strip may be in the thickness range of0.9 to 2.0 mm, and may be in the range of 1.0-1.5 mm. Carbon levels maybe in the range of 0.30-0.60% in certain applications for steel strip.These high carbon steel strip products are achieved without multipleannealing and cold rolling required in past steel compositions toachieve the same properties, although for the present steel furtherprocessing may be desired for certain applications.

We have found that greater than 0.25% carbon steels exhibit a broadertemperature range for the mushy zone of liquid in solid shells incasting the thin strip than is the case in plain low carbon steels. Thebroader range of mushy zone temperatures may require casting rollmodifications for increasing the heat transfer as well as to the shapeof the roll. The composition of the heats studied can be seen in TABLE1.

TABLE 1 Steel C Mn Si Nb V N (ppm) Base 0.02-0.05 0.7-0.9 0.15-0.30<0.003 <0.003 35-90 C—Mn Com- 0.19 0.94 0.21 <0.003 <0.003 85 parative K0.46 0.89 0.20 <0.003 <0.003 95

The composition of Steel K had a free oxygen content of 35.2 ppm, andwas within the ranges of 5 to 50 ppm or of 25 and 45 ppm. The totaloxygen was more than 20 ppm and typically less than 100 ppm. Thecomparative 0.19% carbon steel had a free oxygen content of 37.6 ppm.

The composition of the comparative 0.19% carbon steel is given inTABLE 1. The microstructures obtained in the 0.19% carbon comparisonsteel were complex and are shown for comparison in FIGS. 3A and 3B. Themajority of the microstructure was acicular ferrite with a smallproportion of grain boundary ferrite and some very fine pearlite. Theferrite volume fraction is higher in FIG. 3B, reflecting the higherdegree of hot reduction.

By contrast, the tensile properties of the 0.46% carbon steel of thepresent hot rolled steel strip are presented in TABLE 2, with strengthas cast (1.81 mm thick) and as rolled (1.26 mm thick). The tensilestrengths recorded were over 800 MPa. Some examples of themicrostructure obtained in the as-hot rolled condition are shown inFIGS. 4A and 4B. The samples of the 0.46% carbon steel shown in FIGS. 4Aand 4B and TABLE 2 were processed without water cooling being applied inthe accelerated cooling section of the run out table (i.e. the strip isair cooled).

TABLE 2 Thickness Yield Strength Tensile Strength % Grade (mm) (MPa)(MPa) Elongation 0.46% C 1.81 596.9 835.5 7.3 0.46% C 1.26 587.1 874.813.8

The microstructures, shown in FIGS. 4A and 4B contained very littlepolygonal ferrite as in the comparative steel. The microstructureincludes thin, discontinuous networks of grain boundary ferrite, oftenassociated with very fine feathers of widmanstatten ferrite. The highlevel of hardenability imparted by this carbon content and a smalldifference between A₃ and A₁ temperatures, limited the growth offerrite. The majority of the final microstructure consists of pearlitewith a very fine lamellar spacing, identified using via TEM microscopy,shown in FIG. 5. Measurements of interlamellar spacing between theplates ranged from ˜50-150 nm. In addition to the pearlite,intergranular acicular ferrite was present, showing a thin lenticularstructure.

The hot rolled steel strip may be coiled at a temperature between about550 and 750° C. to provide a majority of the microstructure comprisingpearlite, along with bainite and acicular ferrite in the microstructure.Alternatively, the hot rolled steel strip may be coiled at a temperaturebetween about 200 and 550° C. to provide a majority of themicrostructure comprising bainite, martensite, and acicular ferrite,with less than 5% polygonal ferrite in the microstructure.

The hot rolled steel composition may comprise, by weight, greater than0.25% and up to 1.1% carbon, between 0.4 and 2.0% manganese, between0.05 and 0.50% silicon, less than 0.01% aluminum. The manganese contentmay be between 0.4 and 2.0% by weight, or between 0.4 and 0.7%, orbetween 0.6 and 0.9%, or between 0.7 and 1.0% by weight. Additionally,the molten steel cast may have a free oxygen content between 5 and 50ppm or between 25 and 45 ppm. Total oxygen content may be more than 20ppm and typically less than 100 ppm.

Samples of the present steel were prepared with coiling temperaturesbetween about 640 and 710° C. and carbon amount between about 0.3% and0.4%, shown in FIGS. 6 through 8. Shown in FIG. 6 are tensile strength,yield strength and total elongation over different levels of carbonamount between 0.3% and 0.4%.

The effect of hot reduction on yield strength, tensile strength, andtotal elongation in the present high carbon steels results in a steelproperties where the tensile strength, yield strength and totalelongation are relatively stable with different levels of hot reduction,as shown in FIG. 7. In previous such steel products, there is typicallya decrease in yield and tensile strengths with increasing hot reduction.In contrast, the effect of different amounts of hot reduction on yieldstrength, tensile strength, and total elongation is significantlyreduced in the present steel products. As shown in FIG. 7, the presenthigh carbon steel is relatively stable with the degree of hot rolledreduction for reductions up to at least 25%. Alternatively, the presenthigh carbon steel is relatively stable with the degree of hot rolledreduction for reductions up to at least 35%. The mechanical propertiesat 10% and 35% reduction are within 10% for yield strength, tensilestrength and total elongation. Alternatively, the mechanical propertiesare within 10% throughout the range from 10% to 35% reduction for yieldstrength, tensile strength and total elongation. In yet anotheralternative, the mechanical properties at 15% and 35% reduction arewithin 10% for yield strength, tensile strength and total elongation.Alternatively, the mechanical properties are within 10% throughout therange from 15% to 35% reduction for yield strength, tensile strength andtotal elongation.

As shown in FIG. 8, the tensile strength, yield strength and totalelongation are relatively stable with different coiling temperaturesbetween about 640 and 710° C. (1180 and 1300° F.). Alternatively, thepresent high carbon steel is relatively stable with the coilingtemperature between about 550 and 750° C.

The molten steel may have a composition such that the composition of thehot rolled steel composition comprises a niobium content between about0.01% and 0.2%. The composition may alternatively or in additioncomprise at least one element selected from the group consisting ofmolybdenum between about 0.05% and about 0.50%, vanadium between about0.01% and about 0.20%, and a mixture thereof.

While the invention has been illustrated and described in detail in theforegoing drawings and description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly illustrative embodiments thereof have been shown and described, andthat all changes and modifications that come within the spirit of theinvention described by the following claims are desired to be protected.Additional features of the invention will become apparent to thoseskilled in the art upon consideration of the description. Modificationsmay be made without departing from the spirit and scope of theinvention.

1. A hot rolled steel strip made by the steps comprising: assembling aninternally cooled roll caster having laterally positioned casting rollsforming a nip between them, and forming a casting pool of molten steelsupported on the casting rolls above the nip and confined adjacent theends of the casting rolls by side dams, the molten steel of suchcomposition that hot rolled thin cast strip produced has a compositioncomprising by weight, greater than 0.25% and up to 1.1% carbon, between0.5 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01%aluminum, counter rotating the casting rolls to solidify metal shells onthe casting rolls as the casting rolls move through the casting pool,and forming from the metal shells downwardly through the nip between thecasting rolls a steel strip, hot rolling the steel strip such thatmechanical properties at 10% and 35% reduction are within 10% for yieldstrength, tensile strength and total elongation; and coiling the hotrolled steel strip at a temperature between 550 and 750° C. to provide amajority of the microstructure comprising pearlite, along with bainiteand acicular ferrite in the microstructure.
 2. The hot rolled steelstrip as claimed in claim 1, the hot rolled steel strip such thatmechanical properties at 15% and 35% reduction are within 10% for yieldstrength, tensile strength and total elongation.
 3. The hot rolled steelstrip as claimed in claim 1, the molten steel having a free oxygencontent between 5 and 50 ppm.
 4. The hot rolled steel strip as claimedin claim 1, the molten steel having a free oxygen content between 25 and45 ppm.
 5. The hot rolled steel strip as claimed in claim 1 where thesteel strip has a thickness of less than 2.5 mm.
 6. The hot rolled steelstrip as claimed in claim 1, the molten steel having a composition suchthat the manganese content of the composition of the hot rolled steelstrip is between 0.6 and 1.0% by weight.
 7. The hot rolled steel stripas claimed in claim 1, the molten steel having a composition such thatthe niobium content of the composition of the hot rolled steel strip isbetween about 0.01% and 0.2%.
 8. The hot rolled steel strip as claimedin claim 1, the composition further comprising at least one elementselected from the group consisting of molybdenum between about 0.05% andabout 0.50%, vanadium between about 0.01% and about 0.20%, and a mixturethereof.
 9. The hot rolled steel strip as claimed in claim 1 made by thesteps further comprising the step of: hot dip coating the hot rolledsteel strip to provide a coating of zinc or a zinc alloy.
 10. The hotrolled steel strip as claimed in claim 1 having a tensile strength of atleast 800 MPa after hot rolling reductions of 15% and 35%.
 11. A hotrolled steel strip made by the steps comprising: assembling aninternally cooled roll caster having laterally positioned casting rollsforming a nip between them, and forming a casting pool of molten steelsupported on the casting rolls above the nip and confined adjacent theends of the casting rolls by side dams, the molten steel of suchcomposition that hot rolled thin cast strip produced has a compositioncomprising by weight, greater than 0.25% and up to 1.1% carbon, between0.5 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01%aluminum, counter rotating the casting rolls to solidify metal shells onthe casting rolls as the casting rolls move through the casting pool,and forming from the metal shells downwardly through the nip between thecasting rolls a steel strip, hot rolling the steel strip such thatmechanical properties at 10% and 35% reduction are within 10% for yieldstrength, tensile strength and total elongation; and coiling the hotrolled steel strip at a temperature between 200 and 550° C. to provide amajority of the microstructure comprising bainite, martensite, andacicular ferrite, and having less than 5% polygonal ferrite in themicrostructure.
 12. The hot rolled steel strip as claimed in claim 11,the hot rolled steel strip such that mechanical properties at 15% and35% reduction are within 10% for yield strength, tensile strength andtotal elongation.
 13. The hot rolled steel strip as claimed in claim 11,the molten steel having a free oxygen content between 5 and 50 ppm. 14.The hot rolled steel strip as claimed in claim 11, the molten steelhaving a free oxygen content between 25 and 45 ppm.
 15. The hot rolledsteel strip as claimed in claim 11 where the steel strip has a thicknessof less than 2.5 mm.
 16. The hot rolled steel strip as claimed in claim11, the molten steel having a composition such that the manganesecontent of the composition of the hot rolled steel strip is between 0.6and 1.0% by weight.
 17. The hot rolled steel strip as claimed in claim11, the molten steel having a composition such that the niobium contentof the composition of the hot rolled steel strip is between about 0.01%and 0.2%.
 18. The hot rolled steel strip as claimed in claim 11, thecomposition further comprising at least one element selected from thegroup consisting of molybdenum between about 0.05% and about 0.50%,vanadium between about 0.01% and about 0.20%, and a mixture thereof. 19.The hot rolled steel strip as claimed in claim 11 made by the stepsfurther comprising the step of: hot dip coating the hot rolled steelstrip to provide a coating of zinc or a zinc alloy.
 20. The hot rolledsteel strip as claimed in claim 11 having a tensile strength of at least800 MPa after hot rolling reductions of 15% and 35%.
 21. The hot rolledsteel strip as claimed in claim 11 having a tensile strength betweenabout 1100 and 1400 MPa after hot rolling reductions of 15% and 35%. 22.The hot rolled steel strip as claimed in claim 11 having a tensilestrength between about 1400 and 1700 MPa after hot rolling reductions of15% and 35%.
 23. A method of making hot rolled steel strip, the stepscomprising: assembling an internally cooled roll caster having laterallypositioned casting rolls forming a nip between them, and forming acasting pool of molten steel supported on the casting rolls above thenip and confined adjacent the ends of the casting rolls by side dams,the molten steel of such composition that hot rolled thin cast stripproduced has a composition comprising by weight, greater than 0.25% andup to 1.1% carbon, between 0.5 and 2.0% manganese, between 0.05 and0.50% silicon, less than 0.01% aluminum, counter rotating the castingrolls to solidify metal shells on the casting rolls as the casting rollsmove through the casting pool, and forming from the metal shellsdownwardly through the nip between the casting rolls a steel strip, hotrolling the steel strip such that mechanical properties at 10% and 35%reduction are within 10% for yield strength, tensile strength and totalelongation; and coiling the hot rolled steel strip at a temperaturebetween 550 and 750° C. to provide a majority of the microstructurecomprising pearlite, along with bainite and acicular ferrite in themicrostructure.
 24. The method of making hot rolled steel strip asclaimed in claim 23, the step of hot rolling the steel strip such thatmechanical properties at 15% and 35% reduction are within 10% for yieldstrength, tensile strength and total elongation.
 25. The method ofmaking hot rolled steel strip as claimed in claim 23, the molten steelhaving a free oxygen content between 5 and 50 ppm.
 26. The method ofmaking hot rolled steel strip as claimed in claim 23, the molten steelhaving a free oxygen content between 25 and 45 ppm.
 27. The method ofmaking hot rolled steel strip as claimed in claim 23 where the steelstrip has a thickness of less than 2.5 mm.
 28. The method of making hotrolled steel strip as claimed in claim 23, the molten steel having acomposition such that the manganese content of the composition of thehot rolled steel strip is between 0.6 and 1.0% by weight.
 29. The methodof making hot rolled steel strip as claimed in claim 23, the moltensteel having a composition such that the niobium content of thecomposition of the hot rolled steel strip is between about 0.01% and0.2%.
 30. The method of making hot rolled steel strip as claimed inclaim 23, the composition further comprising at least one elementselected from the group consisting of molybdenum between about 0.05% andabout 0.50%, vanadium between about 0.01% and about 0.20%, and a mixturethereof.
 31. The method of making hot rolled steel strip as claimed inclaim 23 further comprising the step of: hot dip coating the hot rolledsteel strip to provide a coating of zinc or a zinc alloy.
 32. The methodof making hot rolled steel strip as claimed in claim 23 having a tensilestrength of at least 800 MPa after hot rolling reductions of 15% and35%.
 33. A method of making hot rolled steel strip, the stepscomprising: assembling an internally cooled roll caster having laterallypositioned casting rolls forming a nip between them, and forming acasting pool of molten steel supported on the casting rolls above thenip and confined adjacent the ends of the casting rolls by side dams,the molten steel of such composition that hot rolled thin cast stripproduced has a composition comprising by weight, greater than 0.25% andup to 1.1% carbon, between 0.5 and 2.0% manganese, between 0.05 and0.50% silicon, less than 0.01% aluminum, counter rotating the castingrolls to solidify metal shells on the casting rolls as the casting rollsmove through the casting pool, and forming from the metal shellsdownwardly through the nip between the casting rolls a steel strip, hotrolling the steel strip such that mechanical properties at 10% and 35%reduction are within 10% for yield strength, tensile strength and totalelongation; and coiling the hot rolled steel strip at a temperaturebetween 200 and 550° C. to provide a majority of the microstructurecomprising bainite, martensite, and acicular ferrite, and having lessthan 5% polygonal ferrite in the microstructure.
 34. The method ofmaking hot rolled steel strip as claimed in claim 33, the hot rolledsteel strip such that mechanical properties at 15% and 35% reduction arewithin 10% for yield strength, tensile strength and total elongation.35. The method of making hot rolled steel strip as claimed in claim 33,the molten steel having a free oxygen content between 5 and 50 ppm. 36.The method of making hot rolled steel strip as claimed in claim 33, themolten steel having a free oxygen content between 25 and 45 ppm.
 37. Themethod of making hot rolled steel strip as claimed in claim 33 where thesteel strip has a thickness of less than 2.5 mm.
 38. The method ofmaking hot rolled steel strip as claimed in claim 33, the molten steelhaving a composition such that the manganese content of the compositionof the hot rolled steel strip is between 0.6 and 1.0% by weight.
 39. Themethod of making hot rolled steel strip as claimed in claim 33, themolten steel having a composition such that the niobium content of thecomposition of the hot rolled steel strip is between about 0.01% and0.2%.
 40. The method of making hot rolled steel strip as claimed inclaim 33, the composition further comprising at least one elementselected from the group consisting of molybdenum between about 0.05% andabout 0.50%, vanadium between about 0.01% and about 0.20%, and a mixturethereof.
 41. The method of making hot rolled steel strip as claimed inclaim 33 made by the steps further comprising the step of: hot dipcoating the hot rolled steel strip to provide a coating of zinc or azinc alloy.
 42. The method of making hot rolled steel strip as claimedin claim 33 having a tensile strength of at least 800 MPa after hotrolling reductions of 15% and 35%.
 43. The method of making hot rolledsteel strip as claimed in claim 33 having a tensile strength betweenabout 1100 and 1400 MPa after hot rolling reductions of 15% and 35%. 44.The method of making hot rolled steel strip as claimed in claim 33having a tensile strength between about 1400 and 1700 MPa after hotrolling reductions of 15% and 35%.